countdown timer using php and javascript.

<?php
 $date=date_create("2018-12-07 13:06:01");
date_add($date,date_interval_create_from_date_string("30 days"));
// $listing_expire_time= date_format($date,"Y-m-d H:i:s");

 $listing_expire_time= date_format($date,"M j, Y H:i:s");

  //echo $today = date("M j, Y g:i a");             

?>
<p id="demo"></p>

<script>
var countDownDate = new Date("<?php echo $listing_expire_time; ?>").getTime();
var x = setInterval(function() {
var now = new Date().getTime();
var distance = countDownDate - now;
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
   document.getElementById("demo").innerHTML = days + "d " + hours + "h "
    + minutes + "m " + seconds + "s ";
   
  if (distance < 0) {
        clearInterval(x);
        document.getElementById("demo").innerHTML = "EXPIRED";
    }
}, 1000);
</script>